Field of the Invention
The present invention relates to data communications systems, where circuit complexity considerations, processing computation considerations, or DC power considerations significantly constrain available solutions. More specifically, the invention applies to data communication links where multi-path interference is a significant source of performance degradation. In particular, the present invention applies to data communications systems operating through a multi-path acoustic channel. Circumstances involving multi-path acoustic data communications channels include medical applications wherein devices are inserted into animals or humans for diagnostic or therapeutic reasons.
Background Art
Modern data communications systems attempt to optimize bandwidth utilization efficiency by deploying complex modulation and encoding schemes of ever increasing complexity. For example, a radio frequency (RF) wireless data communications system using a 64-QAM (Quadrature Amplitude Modulation) modulation scheme achieves a theoretical efficiency of 6 bits per second per Hertz (bps/Hz), and a typical realized efficiency of 3 to 4 bps/Hz. To achieve that optimization, circuit complexity, computation processing requirements, and transmitted power are not significant constraints on the systems designer and therefore the ultimate bandwidth efficiency utilization can be realized.
However, in certain situations, circuit complexity, computational complexity, and the available levels of transmitted power are significant limitations. Examples of such situations are small remote sensor packages, e.g. ingestible medical diagnostic pills for use in animals and humans. In such situations, modest circuit complexity, reduced computation complexity, and limited transmitted power levels are key system design considerations.
Other examples not served well by conventional solutions are situations characterized by significant multi-path propagation such as that caused by multiple reflections from surrounding objects within the channel environment. Such multi-path propagation causes inter-symbol interference (ISI) and signal fading in the time and frequency domains. Thus, ISI and deep fading (such as Rayleigh fading) significantly decrease system performance, restrict data rate, dramatically increase the necessary power, and at times can lead to the complete degradation of data transmission. Examples of such multi-path propagation environments include RF wireless links in building environments, and acoustic transmission channels in proximity to multiple surfaces, areas and boundaries.
In conventional data communications systems, the challenge of multi path propagation interference is mitigated by the utilization of OFDM (Orthogonal Frequency Division Multiplexing) technology, whose underlying approach capitalizes on the parallel transmission of data by several sub-carriers with a large symbol duration together with a guard interval. However, the conventional OFDM technology is very complex, since it is based on FFT (Fast Fourier Transform) algorithms and PSK (phase shift keying) modulation. FFT algorithms require substantial real-time computations, and PSK modulation requires the use of additional circuitry to generate special pilot signals and for sub-carrier recovery.
What is needed is an approach that solves one or more challenges in a data communications system, namely reduced circuit complexity, reduced computational processing, reduced transmitted power requirements, and improved handling of multi-path interference.